I just read your updates, and ouch. Cupcakes? Seriously? That would definitely be hurtful to me.
I really don't have any advice for you, because I feel like I am reaching in the dark half the time in my own situation, but I think you going dark sounds like it would be appropriate in this situation. I think I would have to do that. Dealing with him hanging out with another woman and still living in your house has to be the hard! He literally wants to "cake-eat".
Take some time for yourself to think hard about where your lines are at and what you are prepared to deal with before you make any major moves, but I'd say he is forcing your hand a little with the way he is living like a single guy and going back and forth with your kids between your houses.
M 36 H 36 D9,S6, Expecting D in August 2012 M 13 T 18 Told me he wanted to separate 12/31/2011 Moved Out 2/2/2012 Didn't want to work on things, wanted divorce 4/20/2012